I'm starting this little project, since I've got all the pieces. The goal is to make a legal, bolt-on addition to a kid's bike - in this case my daughters 26" wheel mountain bike. Georgia allows 500 W and still calls it a bike - so twin 250W permanant magnet brushed motors.

FrictionFast concept

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The thought is to pick up the rack mounts for the main mounts. The mechanical approach is to pivot the motor plate (green) on a different center than the rear wheel axle. This way, drive torque from the motors tries to push the motor toward the rear of the bike, and it presses more tightly into the tire - ideally, the more torque the more pressure.

FrictionFast "Engaged"

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Pull the motor mount forward, against the seat stays, and the drive rollers lift clear of the tire.

FrictionFast - "Idle"

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The motors I'll run at 48 volts, and max out at 6000 rpm - that's what I have them doing on scooters now - no, I know they won't be running at 250 Watts - but that's what the labels say

I'll use 35-durometer urethane "cushion" rollers (they have teardrop shaped voids through the roller) from Fairlane to start.

I'm very interested in hearing more about those urethane rollers, are they textured or just smooth? If I understand correctly, they are designed to deform and take the shape of the wheel under power? How well do you expect they will grip the tire if things get wet?

When the roller is resting forward and off of the tire, how will it engage? Its my understanding that the Kepler/Adrian style wants to jerk backwards when turned on (a phenomenon I noticed when operating a hand-drill), and of course, only a tiny movement is needed to get the roller to touch the tire, and then begin pulling itself deeper into the tread.

The thought occurred to me that if the roller was resting on top of a stationary plate (both not touching the tire), turning on the motor would impart some movement, and as stated before, only a small amount of momentum is needed to get the assembly moving towards tire-engagement. This looks like a fun project, and I wish you the best of luck with whatever you decide.

edit: I think that this is a very clever design, and once you factor in that a variety of motors could be easily optioned-in (brushless, higher power if desired), this style has a lot going for it. Since it doesn't require one-way clutch-bearings, that is one less part, one less possible failure/wear concern, and the roller choices are much better.

Have you seen the posts about "stippling" metal? A simple shaft union (soft steel) could connect the two motor shafts and form not only the connection, but an affordable steel roller. Stipple the roller skin to give it a rough surface, then heat it and roll in sugar (Jeremy's suggestion) to harden the skin. The roller skin could also be cheaply knurled by a shop (rather than stippled), and most local fab shops have knurling jigs. I look forward to hearing how the urethane holds up. That may prove to be a great roller solution...

Last edited by spinningmagnets on Jan 29 2011 7:50pm, edited 1 time in total.

At this point, I don't know what will make it engage - have to see if the "jerk" will do it - it seems like it would kick the swing plate forward, but maybe it could bounce of a cushion and back into the tire? If not, maybe some sort of lever thingy - Or hey - just had an idea - what about a little tab just under the roller at rest position, but clearing the tire? It could serve as the "launch platform".

Battery box cardboard model - this fit the battteries correctly, but not the bike (hits the stem).

First battery box model

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I'll fudge the next model to fit the frame angles exactly, and think the batteries will fit fine. That cardboard is BCC flute, or 1/4" thick, while the actual battery box will be .063 steel, so that will save me almost 1/2" all around.

Wow, thats a awefully large battery case for a low power friction drive. But your concept will work and you might want to look into some way of dampening the engagement and disengagement of the drive as with Kelpers drive, the commuter friction drive Adrian is doing, both mounting from above the wheel and the one i built(similar to yours mounting from below) all the drives have a dampening system in them to stop the kick back.

Yep, all those things I've got to do. The bike (a kid's frame) is pretty short, so I can straddle the battery box fine.

In regards to the battery box size, I really don't know what capacity I need - four 12V packs seemed not enough, eight does look big, but 48 Volts is what I'm using on the kids' scooters. If 36 V would work, I could use six packs.

Power-wise, I'll probably use a 40 or 60 A controller, just because I've got some. Max power should be about 2KW, just based on my experience so far with this stuff. I have ordered a couple of those $30 wattmeters from Asia, so hopefully I'll know for real what's happening with the electrons.

This is all sort of an experiment to learn, using all the material I have.

Forrest... I'm interested in your drive ,as it is very much like my drive in the way it contacts the tire ..( Hill Helper thread ) I just want to say I have been experimenting with what I call a park ramp to get the roller off the tire .. As mentioned above by Spinner.. I found that it works , but when not calling for more torque, or when coasting, you end up on the edge of the ramp Needless to say not good for the roller ... Thats been my experence with my drive .. Yours could be different , because it is working on top of the wheel ,and mine has to go up the tire in front .. Good luck with your build, It's looking good ...

McDesign wrote:
Power-wise, I'll probably use a 40 or 60 A controller, just because I've got some. Max power should be about 2KW,
This is all sort of an experiment to learn, using all the material I have.

I would recommend that you invest a little time & money in a brake up-grade before you experiment too much.
..Those calipers are going to work overtime holding back 2kw and all those batteries !
Minimum of a front V brake, though a disc may be easier.

Got my little axles made that join the two motors and onto which the roller mounts. I sent a perfectly fine piece of mild steel 7/8" rod to my way-back-from-college buddy Bruce - http://www.boonerings.com/ , but he didn't like it - seemed "crude". He made these out of ring-quality Titanium, 'cuz that's what he's set up for!

Titanium axles

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Y'all need a ring of any type for any thing, he's your man - goal is a week turn-around, always - check out his site. He made lots of Ti bike parts, long ago, and some great HPVs - I think still 83rd fastest in the world, ever, in about 1988.

Hey - had to play tonight after work! Everything fit very nicely together - so tightly that I had to polish a bit - no runout at all.

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The inner setscrew on each end of the titanium axle engages the normal flat on the 8mm motor shafts, the outer is a backup. the two setscrews on the roller hub just engage the axle - I'll mark the places, and file a small flat for the setscrew to engage.